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Re: basic asm and memory clobbers


On Fri, Nov 20, 2015 at 04:29:50PM +0100, Richard Henderson wrote:
> On 11/20/2015 04:20 PM, Segher Boessenkool wrote:
> >On Fri, Nov 20, 2015 at 02:05:01PM +0100, Richard Henderson wrote:
> >>I'd be perfectly happy to deprecate and later completely remove basic asm
> >>within functions.
> >>
> >>Because IMO it's essentially useless.  It has no inputs, no outputs, and no
> >>way to tell the compiler what machine state has been changed.  We can say
> >>that "it clobbers everything", but that's not actually useful, and quite
> >>difficult as you're finding out.
> >>
> >>It seems to me that it would be better to remove the feature, forcing what
> >>must be an extremely small number of users to audit and update to extended
> >>asm.
> >
> >Should  asm("bla");  then be an extended asm with no input, no outputs,
> >no (non-automatic) clobbers?  That would be the most straightforward and
> >logical semantics, but will it break user code?
> 
> I'm suggesting that we don't accept that at all inside a function.  One must
> audit the source and make a conscious decision to write asm("bla" : );
> instead.
> 
> Accepting basic asm outside of a function is perfectly ok, since that's just
> a mechanism by which one can inject complete assembly routines into a C
> translation unit.

Isn't that going to break too much code though?  I mean, e.g. including
libgcc...

	Jakub


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